1. Field of the Invention
The present invention relates to electronic switching power supplies and specifically to switched mode power supplies operating in a critical conduction mode.
2. Description of the Prior Art
Switched mode power supplies operating in a critical conduction mode are known in the art. In the critical conduction mode first a current through the primary winding of the transformer of the power supply is switched on. Depending on the load on the secondary side the primary current is switched off when a peak current is reached. In response to this the energy of the electromagnetic field of the transformer is transferred to its secondary side to supply power to the load. After all the electromagnetic energy has been transformed the current on the primary side of the transformer is switched on again to start the same sequence.
With reference to FIG. 1 in which such a prior art power supply is shown this is explained in more detail. Power supply 10 has a transformer 11 which is coupled to a high voltage source. Primary winding 12 of the transformer 11 is connected to power transistor 13. If the power transistor 13 is switched on a primary current 14 starts to build up. The primary current 14 is sensed by current sensor 15. The current sensor 15 has its output 16 connected to controller 17. The controller 17 has its control output 19 coupled to gate 18 of the power transistor 13. Via the control output 19 of the controller 17 the power transistor 13 is switched on and off.
The transformer 11 has secondary winding 20 to which a load is connected. The load is not shown in the drawing--typically it can be a low power device such as a battery charger for a hand held device, in particular for a cellular telephone or radio. The load which is placed on the secondary winding 20 is sensed by load sensor 22 having an output 23. The output 23 of the load sensor 22 is coupled to input 24 of the controller 17. The signal delivered by the load sensor 22 is representative of the load placed on the secondary winding 20. Typically conductive coupling between the secondary and primary side is prevented by making usage of opto-couplers which is well known in the art.
Further the transformer 11 has an auxiliary winding 21 on its secondary side. The current flow through the auxiliary winding 21 is monitored by current sensor 25 which is connected to the auxiliary winding 21. The current sensor 25 has an output 26 which is coupled to input 27 of the controller 17.
The controller 17 has a control logic 28 which is coupled to the output 19 and the inputs 16, 24 and 27. Further the controller 17 comprises a so-called frequency clamp 29 which is a timer circuit. Output 30 of the frequency clamp 29 is connected to the control logic 28. The control logic 28 generates the control output 19. The control output 19 is also coupled internally the controller 17 to the frequency clamp 29.
In operation the controller 17 determines the peak level of the primary current 14 which depends on the load sensed by load sensor 22. The higher the load, the higher the required peak primary current 14. When the peak primary current 14 is reached the control logic 28 issues an output signal at its control output 19 to switch the power transistor 13 off. This output signal of the control logic 28 is also received by the frequency clamp 29 and starts the timer. After a certain time delay of typically in the order of 6 milliseconds the frequency clamp 29 issues an output signal at its output 30 to the control logic 28.
When the primary current 14 is switched off this results in a transfer of the electromagnetic energy stored in the transformer 11 to the secondary winding 20. The condition of the transformer 11 is sensed by the auxiliary winding 21. After the field energy of the transformer 11 has been consumed by the load, the voltage on the auxiliary winding 21 decreases in a damped oscillation. When the current through the auxiliary winding 21 drops below a certain level this results in an output signal issued by current sensor 25 at its output 26 to the control logic 28. Because of the damped oscillation there can be a sequence of such output signals.
The control logic 28 issues an output signal at its control output 19 to switch the power transistor 13 on again if both of the following conditions are fulfilled:
The time delay determined by the frequency clamp after the switching off of the power transistor 13 has expired which is signaled by the output signal issued by the frequency clamp 29; and PA1 an output signal is received from the current sensor 25 which indicates a zero current condition on the secondary side of the transformer 11.
One of the problems associated with prior art power supplies of the above described type is that the switching on of the power transistor 18 is sometimes delayed. One of the consequences is that the peak primary current 14 has to be set to a relatively high level.
The invention is therefore aimed at providing an improved switched mode power supply and an improved method for controlling a switched mode power supply.